1. Field of the Invention
The present invention relates generally to the field of geophysical exploration apparatus. More specifically, the present invention is related to systems for acquiring seismic data from a plurality of sensors and telemetering the signals acquired by the sensors to a recording unit.
2. Discussion of the Related Art
Seismic exploration is used to determine the presence of subsurface structures in earth formations which may contain useful materials such as petroleum. Seismic exploration methods known in the art include positioning a plurality of acoustic sensors, which can be geophones, at spaced apart locations in a predetermined pattern on the surface of the earth. The earth is then energized by an acoustic energy source typically located at a predetermined position in the vicinity of the geophones. The acoustic energy from the source radiates generally downwardly into the earth. Some of the acoustic energy can be partially reflected back towards the earth's surface by subsurface acoustic impedance boundaries, called reflectors, which may exist within the earth. The reflected acoustic energy can be detected by the geophones. The geophones generate electrical signals proportional to the magnitude of the acoustic energy.
In a typical seismic survey, the electrical signals are digitized, or converted into series of numbers representing the magnitudes of the electrical signals sampled at spaced-apart time intervals. The digitized signals are then typically stored or recorded in a recording unit.
What is of particular interest in seismic exploration is the time, relative to the instant at which the acoustic energy is imparted into the earth by the source, at which each signal from each geophone is generated. Maps denoting the time of arrival of reflected energy at each geophone can be generated from the digitized signals from each geophone. The maps can be further processed by methods known in the art into maps describing the subsurface structures.
It is known in the art to provide a very large number of geophones, sometimes as many as six thousand, and in some instances to provide a plurality of acoustic energy sources, to record a typical seismic survey. As is understood by those skilled in the art, it is desirable to include the signals from such a large number of geophones to enhance the ability to generate a detailed map of the reflectors and other subsurface structures within the earth.
It is also desirable, in order to generate more detailed maps of the subsurface, to preserve a substantial portion of the total frequency spectrum, or bandwidth, of the electrical signals generated by the geophones. As is also understood by those skilled in the art, preserving a substantial portion of the bandwidth during the process of digitizing the electrical signals requires generating digital samples at a rate corresponding to the uppermost frequency to be preserved. In a typical seismic survey, the digital sampling rate can be about 2 milliseconds, or 500 Hz, which can preserve frequencies of up to 250 Hz. Further, each typical digital signal sample comprises twenty-four individual digital "bits". In order to transfer serially the bits in each sample before another sample is taken, that sample must be transmitted to the recording unit at a frequency of no less than 12,000 bits/second (obtained by: 500 Hz/sample/second.times.24 bits/sample), multiplied by the number of sensors used in the survey. Due to practical electronic circuit limitations, the data must be transmitted at a much higher frequency than determined by the previously described formula.
It is known in the art to provide a telemetry system which generates the digital signal samples at each of the geophones and transmits the samples to the recording unit in a serial format on a single, high frequency telemetry line. Because of the very large number of sensors provided in a typical seismic survey, extending the single, high frequency telemetry line known in the art out to the sensors most distant from the recording unit can be difficult and expensive.
It is also known in the art to provide a plurality of high frequency telemetry units near groups of sensors each having digitizing equipment to reduce dam transmission delays and relieve the telemetry burden on the single, high frequency telemetry line. Using multiple, high frequency telemetry units has the drawbacks of requiring both a large number of high frequency telemetry units and requiring a very large amount of interconnecting cable to connect the high dam rate telemetry units to the recording unit. It can be difficult, expensive, and can consume a large amount of electrical power to operate a large number of the high frequency telemetry units near groups of sensors.
Another drawback to the system known in the art for transferring digitized sensor signals to the recording unit is that during transmission of data to the recording unit the system known in the art typically requires a period of inactivity, called "deadtime", in which no new signals can be acquired or digitized, and consequently no acoustic energy can be imparted to the earth to survey. The system dead-time can cause the system known in the art to consume an excessive amount of time to conduct a seismic survey.
Yet another drawback of the seismic recording system known in the art is that the individual sensor digitizing units and the sensors cannot be individually controlled and tested from the recording unit. Accordingly, problems which may occur in a portion of the survey equipment are difficult and time consuming to isolate.
It is an object of the present invention to provide a seismic acquisition system having serially-linked very high rate telemetry units positioned near the sensor digitizing units to reduce the dead-time of the survey system with a minimum of additional interconnecting cabling.
It is a further object of the present invention to provide a seismic acquisition system having very high data rate telemetry units which can be serially interconnected in order to provide greater geometric flexibility in deployment of geophones on the earth's surface.
It is still a further object of the present invention to provide a seismic acquisition system having sensor digitizing units which are individually controllable and can be tested from the recording unit, or from a high frequency telemetry unit positioned elsewhere in the survey system.
It is yet a further object of the present invention to provide a seismic acquisition system having signal digitizing units which form an integral part of the cables interconnecting the seismic sensors to the recording unit in order to reduce the possibility of loss or destruction of the digitizing units and to reduce the overall size and weight of the acquisition system.